This invention relates to a television signal transmission system and more particularly to a stationary-picture transmission system which is capable of transmitting a television signal over a narrowband transmission line.
In a conventional stationary-picture transmission system such as the one described in an article entitled "Development of Color Freeze-picture Transmission System" published in NEC RESEARCH & DEVELOPMENT, No. 43, October, 1976 pp 1-13 or in the U.S. Pat. No. 4,164,760 entitled "STATIONARY-PICTURE TRANSMISSION SYSTEM UTILIZING A DIGITAL MEMORY TECHNIQUE" issued to Inaba et al. and assigned to the present assignee, a television video signal of one frame or one field produced by a pick-up tube at a television standard scanning rate is stored in a one-frame or one-field memory. The stored signal is read out at a lower scanning rate in order to transmit the signal over a narrowband transmission line such as a public telephone line to a receiving end. At the receiving end, the received signal is stored in a one-frame or one-field memory at the lower scanning rate and then read out at the television standard scanning rate.
The stationary-picture transmission over the public telephone line has been used in a telephone conference system in which a conference is held between persons located at a distance from each other. In the telephone conference system, visual data representing not only the persons but also documents to be used in the conference are transmitted as the narrowband stationary-picture signals. In the document data transmission, it happens that a presently transmitting document is desired to be compared with a previously transmitted document. In this case, it is desired to stop the transmitting document at any time point during one complete picture transmission. In the conventional stationary-picture transmission system, however, the stationary-picture is transmitted in a unit of one complete picture, i.e., one-frame by one-frame. In other words, it is impossible to stop the transmission at any time point within one complete picture transmission.